The one-electron reduction kinetics of the europium cryptates Eu(2.2.1)3+ and Eu(2.2.2)3+ by the aquo ions Vaq 2+ and Euaq 2+ and the oxidation kinetics of Eu(2.2.1)2+ by Co(NH3)6 3+ have been studied by using a polarographic technique in order to examine the effects of encapsulating europium within cryptate cavities upon the reactivity of the Eu(III/II) couple. At 25°C and an ionic strength μ = 0.1, the second-order rate constants (M-1 s-1) for acid-independent pathways are as follows: Eu(2.2.1)3+-Vaq 2+, 0.5; Eu(2.2.1)3+-Euaq 2+, ca. 0.2; Eu(2.2.2)3+-Euaq 2+, 1.5; Eu(2.2.2)3+-Euaq 2+, 1.4; Co(NH3)6 3+-Eu(2.2.1)2+, 0.055. By comparison of these kinetic data with those for similar reactions involving the Euaq 3+/2+ couple, the rate constant for Eu(III/II) self-exchange, kex, is estimated to increase by factors of ca. 1 × 107 and 2 × 104 upon encapsulation of europium in (2.2.1) and (2.2.2) cryptate cavities, respectively. Estimates of kex equal to ca. 10, 4 × 10-2, and 5 × 10-6 M-1 s-1 (μ = 0.1) for Eu(2.2.1)3+/2+, Eu(2.2.2)3+/2+, and Euaq 3+/2+, respectively, are obtained from the Marcus cross relation. The increases in kex resulting from cryptate encapsulation suggest that nonadiabaticity is not primarily responsible for the extremely low reactivity of Euaq 3+/2+. The values of kex are shown to be roughly consistent with the Franck-Condon barriers estimated from structural data.
|Number of pages||6|
|Publication status||Published - 1983|
ASJC Scopus subject areas
- Inorganic Chemistry